Data processing method for segmented fitting of fatigue crack growth rate curve

Abstract

The invention discloses a data processing method for segmented fitting of a fatigue crack propagation rate curve, which belongs to the technical field of metal material fatigue crack propagation rate tests, and comprises the following steps: obtaining a data curve to be fitted, determining the amplitude of a stress intensity factor, and setting the range interval of segmented fitting. Fitting data points in the range interval subjected to piecewise fitting to obtain a linear equation, converting the obtained linear equation into a power function form of a Paris formula, and substituting the stress intensity factor amplitude into the power function form of the Paris formula to calculate the corresponding fatigue crack growth rate. According to the method, the calculated fatigue crack growth rate under the specified stress intensity factor amplitude is closer to a real test value, and the fatigue crack growth rate serving as a material engineering application basis is more reliable.

Description

Technical Field

[0001] The invention discloses a data processing method for segmented fitting of a fatigue crack growth rate curve, and belongs to the technical field of fatigue crack growth rate testing of metal materials. Background Art

[0002] As a lightweight, high-strength and corrosion-resistant material, aluminum alloy is widely used in aerospace, rail transportation, shipbuilding, automobiles and other fields. Fatigue failure is the main way of failure of engineering structural parts. 50% to 90% of the damage to mechanical parts is fatigue failure, and in some areas it can reach 80% to 90%. As the baseline for fatigue performance prediction, aluminum alloy fatigue life prediction test methods have been widely studied in many standards and literature. When the fatigue crack growth rate of the test material is required, the current product specifications usually require the test to specify the fatigue crack growth rate value da / dN corresponding to the stress intensity factor amplitude. For example, the product specification requires the test material to be When ΔK is greater than ΔK, the corresponding da / dN value must not exceed the specified value. However, the current fatigue crack growth rate test method standard does not specify how to obtain the da / dN value. The conventional practice is to fit the entire fatigue crack growth rate curve into a Paris formula and then calculate the fatigue crack growth rate value da / dN corresponding to each ΔK separately. The Paris formula is based on linear elastic fracture mechanics, but the actual crack growth process is not entirely linear elastic. In actual production, only one specimen is generally used to test the da / dN-ΔK curve. Although the curve after taking double logarithmic coordinates is generally linear, using only a single linear equation cannot effectively describe the da / dN values corresponding to all ΔKs in the full range. Summary of the Invention

[0003] The object of the present invention is to provide a data processing method for piecewise fitting of a fatigue crack growth rate curve, so as to solve the problem of inaccurate piecewise fitting of a fatigue crack growth rate curve in the prior art.

[0004] A data processing method for segmented fitting of fatigue crack growth rate curve is used to obtain the data curve to be fitted, determine the stress intensity factor amplitude ΔK, and set the segmented fitting range to is the unit of stress intensity factor, The linear equation obtained by fitting the data points within is transformed into the power function form of the Paris formula, and then ΔK is substituted into the power function form of the Paris formula to calculate the corresponding da / dN value, where da / dN is the fatigue crack growth rate, a is the crack length, and N is the number of stress cycles.

[0005] When ΔK is at the head of the curve to be fitted, if the data before ΔK is insufficient, The minimum value of ΔK is taken as the minimum value of the range interval.

[0006] When ΔK is at the tail of the curve to be fitted, if the data after ΔK is insufficient, The maximum value of ΔK is taken as the maximum value of the range interval.

[0007] when When it is not on the node of the data curve to be fitted, take The node of the first data curve to be fitted starting from the positive infinity direction is taken as the minimum value of the range interval.

[0008] when When it is not on the node of the data curve to be fitted, take The node of the first data curve to be fitted starting from the negative infinity direction is taken as the maximum value of the range interval.

[0009] Compared with the prior art, the present invention has the following beneficial effects: the fatigue crack growth rate under a specified ΔK calculated by the present invention is closer to the actual test value and is more reliable as a basis for material engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Schematic diagram of the fitting interval of the present invention. DETAILED DESCRIPTION

[0011] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention are described clearly and completely below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0012] A data processing method for segmented fitting of fatigue crack growth rate curve is used to obtain the data curve to be fitted, determine the stress intensity factor amplitude ΔK, and set the segmented fitting range to is the unit of stress intensity factor, The linear equation obtained by fitting the data points within is transformed into the power function form of the Paris formula, and then ΔK is substituted into the power function form of the Paris formula to calculate the corresponding da / dN value, where da / dN is the fatigue crack growth rate, a is the crack length, and N is the number of stress cycles.

[0013] When ΔK is at the head of the curve to be fitted, if the data before ΔK is insufficient, The minimum value of ΔK is taken as the minimum value of the range interval.

[0014] When ΔK is at the tail of the data curve to be fitted, if the data after ΔK is insufficient, The maximum value of ΔK is taken as the maximum value of the range interval.

[0015] when When it is not at the node of the data curve to be fitted, take The node of the first data curve to be fitted starting from the positive infinity direction is taken as the minimum value of the range interval.

[0016] when When it is not at the node of the data curve to be fitted, take The node of the first data curve to be fitted starting from the negative infinity direction is taken as the maximum value of the range interval.

[0017] The present invention cuts TL-to-full-thickness M(T) specimens from 6.45mm thick 7050-T7451 aluminum alloy sheets and tests the fatigue crack growth rate curve. The corresponding da / dN values, the range of ΔK data used in the corresponding fitting curve, the corresponding Paris formula parameters, and the corresponding da / dN value calculation results are shown in Table 1, and the fitting interval is shown in Figure 1 From the comparison data in Table 2, as ΔK increases, the relative error of the crack growth rate calculated by the two methods becomes larger. The crack growth rate obtained by this method has a significantly smaller absolute error than the true value of the approach point, and is closer to the true value.

[0018] Table 1 Fitting intervals, Paris formula parameters, and da / dN values corresponding to different target ΔK

[0019]

[0020] Table 2 Crack growth rates obtained by the present invention and the existing method

[0021]

[0022] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents, and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A data processing method for piecewise fitting of fatigue crack growth rate curve, characterized in that: Obtain the data curve to be fitted, determine the stress intensity factor amplitude ΔK, and set the range of the segmented fitting to is the unit of stress intensity factor, The linear equation obtained by fitting the data points within is transformed into the power function form of the Paris formula, and then ΔK is substituted into the power function form of the Paris formula to calculate the corresponding da / dN value, where da / dN is the fatigue crack growth rate, a is the crack length, and N is the number of stress cycles.

2. The data processing method for piecewise fitting of fatigue crack growth rate curve according to claim 1, characterized in that: When ΔK is at the head of the curve to be fitted, if the data before ΔK is insufficient, The minimum value of ΔK is taken as the minimum value of the range interval.

3. The data processing method for piecewise fitting of fatigue crack growth rate curve according to claim 1, characterized in that: When ΔK is at the tail of the curve to be fitted, if the data after ΔK is insufficient, The maximum value of ΔK is taken as the maximum value of the range interval.

4. The data processing method for piecewise fitting of fatigue crack growth rate curve according to claim 1, characterized in that: when When it is not at the node of the data curve to be fitted, take The node of the first data curve to be fitted starting from the positive infinity direction is taken as the minimum value of the range interval.

5. The data processing method for piecewise fitting of fatigue crack growth rate curve according to claim 1, characterized in that: when When it is not at the node of the data curve to be fitted, take The node of the first data curve to be fitted starting from the negative infinity direction is taken as the maximum value of the range interval.

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